What makes a urinary tract infection complicated?

Consider anatomical and severity risk factors



A 72-year-old woman with type 2 diabetes mellitus presents with acute dysuria, fever, and flank pain. She had a urinary tract infection (UTI) 3 months prior treated with nitrofurantoin. Temperature is 102° F, heart rate 112 beats per minute, and the remainder of vital signs are normal. She has left costovertebral angle tenderness. Urine microscopy shows 70 WBCs per high power field and bacteria. Is this urinary tract infection complicated?

University of Kentucky School of Medicine

Dr. Nhi Vu


The urinary tract is divided into the upper tract, which includes the kidneys and ureters, and the lower urinary tract, which includes the bladder, urethra, and prostate. Infection of the lower urinary tract is referred to as cystitis while infection of the upper urinary tract is pyelonephritis. A UTI is the colonization of pathogen(s) within the urinary system that causes an inflammatory response resulting in symptoms and requiring treatment. UTIs occur when there is reduced urine flow, an increase in colonization risk, and when there are factors that facilitate ascent such as catheterization or incontinence.

There are an estimated 150 million cases of UTIs worldwide per year, accounting for $6 billion in health care expenditures.1 In the inpatient setting, about 40% of nosocomial infections are associated with urinary catheters. This equates to about 1 million catheter-associated UTIs per year in the United States, and up to 40% of hospital gram-negative bacteremia per year are caused by UTIs.1

Dr. Adam J. Gray, assistant professor in the University of Kentucky Division of Hospital Medicine and the Lexington VA Medical Center

Dr. Adam J. Gray

UTIs are often classified as either uncomplicated or complicated infections, which can influence the depth of management. UTIs have a wide spectrum of symptoms and can manifest anywhere from mild dysuria treated successfully with outpatient antibiotics to florid sepsis. Uncomplicated simple cystitis is often treated as an outpatient with oral nitrofurantoin or trimethoprim-sulfamethoxazole.2 Complicated UTIs are treated with broader antimicrobial coverage, and depending on severity, could require intravenous antibiotics. Many factors affect how a UTI manifests and determining whether an infection is “uncomplicated” or “complicated” is an important first step in guiding management. Unfortunately, there are differing classifications of “complicated” UTIs, making it a complicated issue itself. We outline two common approaches.

Anatomic approach

A commonly recognized definition is from the American Urological Association, which states that complicated UTIs are symptomatic cases associated with the presence of “underlying, predisposing conditions and not necessarily clinical severity, invasiveness, or complications.”3 These factors include structural or functional urinary tract abnormalities or urinary instrumentation (see Table 1). These predisposing conditions can increase microbial colonization and decrease therapy efficacy, thus increasing the frequency of infection and relapse.

This population of patients is at high risk of infections with more resistant bacteria such as extended-spectrum beta-lactamase (ESBL) producing Escherichia coli since they often lack the natural genitourinary barriers to infection. In addition, these patients more often undergo multiple antibiotic courses for their frequent infections, which also contributes to their risk of ESBL infections. Genitourinary abnormalities interfere with normal voiding, resulting in impaired flushing of bacteria. For instance, obstruction inhibits complete urinary drainage and increases the persistence of bacteria in biofilms, especially if there are stones or indwelling devices present. Biofilms usually contain a high concentration of organisms including Proteus mirabilis, Morgenella morganii, and Providencia spp.4 Keep in mind that, if there is an obstruction, the urinalysis might be without pyuria or bacteriuria.

Instrumentation increases infection risks through the direct introduction of bacteria into the genitourinary tract. Despite the efforts in maintaining sterility in urinary catheter placement, catheters provide a nidus for infection. Catheter-associated UTI (CAUTI) is defined by the Infectious Disease Society of America as UTIs that occur in patients with an indwelling catheter or who had a catheter removed for less than 48 hours who develop urinary symptoms and cultures positive for uropathogenic bacteria.4 Studies show that in general, patients with indwelling catheters will develop bacteriuria over time, with 10%-25% eventually developing symptoms.


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